consider using RAII for RcMut instead of closures

[thestinger]

Closures actually make this simpler, because they enforce stack discipline. Switching to RAII will involve using a counter instead of just pushing/popping one layer of state so we'll need to take a close look at the performance.

[thestinger]

Note that it isn't possible to drop this in favour of a Mut<T> type until #9509 is dealt with.

[thestinger]

RcMut is now gone.